1. Field of the Invention
The present invention generally relates to high density micro miniaturized electronic circuit devices. More particularly, this invention relates to an interposer for establishing electrical connection between components.
2. Background Information
Part of the process of manufacturing Integrated Circuits (ICs) involves testing them to be sure the circuitry of the IC functions as planned. In order to do this, the electrical connections of an IC are electrically connected the test equipment through an interposer. The type of connection between the IC and the test equipment varies with the design of the IC. ICs which have projecting probes or leads are typically contacted by inserting pressure on the leads. Another design of ICs is an IC which has electrical connections on one side of the IC which could best be described as bumps, balls or contact pads and are typically contacted by inserting pressure on the ICs. The leads or bumps are quite small, and must have a clean electrical connection to the test equipment as well as to the circuit in which the IC will function when in use. In high frequency application the thickness of the interposer is critical. The thickness relates to the speed the interposer can transfer electrical signals. This speed of transfer is more critical at high frequencies than at lower frequencies. The thinner the interposer the faster the transfer of electrical signals.
The problem with establishing a clean connection with an IC which has lead or bump type electrodes is that all available conductors of which the electrode can be made are subject to corrosion, and form a thin layer of insulating oxide on the surface. This includes aluminum, copper, nickel, tin, or any other known conductor. (Gold does not form an oxide, and is a good conductor, but it is soft, wears out easily and is expensive). The problem has been to establish an electrical connection for a lead or bump type IC which penetrates the inevitable layer of corrosion to establish a clean electrical connection.
The current technology uses a device called an interposer, which provides an electrical contact between the IC and the test equipment. The interposer is about as big as a large postage stamp, and the electrical footprint of connectors are a group of conductive pads on the interposer, and are typically in a pattern to match the leads or bumps of the IC that is being tested. By contrast, the interposer of the invention creates a contact surface by cutting away material, not by adding material. This results in a thinner interposer, which transmits signals faster.
The conductive surfaces of currently used interposers use conductive regions that are basically a deliberately roughened surface, to punch through the layer of corrosion. The roughened surface is formed of small metallic beads, which are attracted to a region, and fixed in place. The small metallic beads are typically very hard, and are coated with a layer of hard conductive metal such as nickel. When the electrode of an IC is pressed against the region of small metallic beads, the beads act like small needles, and cut through the layer of oxidation. This is achieved by the height of some of the beads being greater than the surrounding matrix, so that more pressure is applied through the highest regions of beads, which enables them to cut through the oxidation. A problem with this method is that the dispersion of metallic beads over the regions of contact is somewhat irregular, and there can be void regions in which there are no metallic beads, and build-up regions in which small stacks or piles of beads make a mini mountain. This is undesirable because the random small stacks over the IC footprint cause some conductors on the IC not to make electrical connection.
My invention is a thin film interconnect between the IC and test equipment, IC and load board, IC and printed circuit assemblies and the method of making it. This thin film interconnect creates a contact surface which will cut through the oxidation layer of the conductive metals, form a clean and predictable electrical connection with ICs pads, will last a long time, not damage electrodes, and forms a solderless interchangeable connection. This surface is made of a copper material, in which shallow grooves are formed, leaving a number of flat-topped peaks on the surface. Although the peaks are flat topped, they are very small in size, and have proven effective at cutting through the oxidization of metal conductors.
The top of the peak is typically flat on top with square sides and the peaks are coplanar. The top of the peak is approximately 0.001xc3x970.001 inches in size. The peaks are typically on centers of 0.003 to 0.006 inches. The valleys between the peaks can be cut using a laser set to a power which ablates but does not penetrate the copper of the electrode. The valleys between the peaks can also be cut using a microsaw, or other physical device. They can also be etched using a number of chemical means which are standard in the industry. The grids thus formed are positioned in a thin film interconnect for placement between the conductors of an IC and a testing or production mounting.
The flat top peaks are cut in a layer of copper which forms a sandwich of copper around an inner layer of insulating flexible material. Thus there are flat top peaks in an upper layer, which face away from the inner layer. There are also flat top peaks in a lower layer which face away from the inner insulating layer. The upper and the lower layer are connected to each other by a number of copper connections, which pass through vias through the insulating layer.